It is often necessary or desirable to change all or part of an image from one color to another. Images may be taken from many sources, such as motion picture film, video cameras, or computers graphics to name a few. Ordinarily, color correction involves minor color changes. For example, it might be desirable to change an apple from one shade of red to another shade of red. However, more extreme examples may arise, such as where it is desired to change an apple from red to green.
In its most basic form, color correction may involve changes to the totality of the image. For example, it is possible to change the black level, or the white level of an image, or to change the gamma, which changes the shape of the curve in between.
Secondary color correction permits changing a particular hue (or color) to another hue. Devices are commercially available for performing such secondary color correction. In the post-production television and film industry, a person who operates a color corrector is called a colorist. Early versions of secondary color correctors permitted changing various components of red, green and blue (RGB) and yellow, cyan and magenta (YCM). More advanced versions permit dividing colors into a fraction of the spectrum, for example, selecting one color of a spectrum divided into 1024 segments, and changing merely that color.
In addition to changing that selected color, it was also possible to select a given box or region within the image, and to affect the color solely within that box or region. In certain cases, the boundaries were made to be fuzzy, thereby providing no sharp demarkation in color correction of the image.
By way of background, colors may be generated by some combination of red, green and blue (RGB). FIG. 1 shows a fully saturated color sweep in which the percentage of blue is shown in small dashed line, the percentage of green in shown in solid line and the percentage of red is shown in larger dashed line. In the fully saturated condition, the hue may be represented by the angle on the horizontal axis. For example, at 60.degree., where red and blue are in equal parts, the resulting color is magenta. Similarly, at 180.degree., where red and green are in equal parts, the color is yellow.
Rather than describing a given color in terms of its RGB signals, an image may be described in terms of different coordinate systems. One widely used coordinate system utilizes the components Y, u and v. The "Y" component describes the luminance, or monochrome brightness, of the color. The chrominance or color information, is contained in the "u" and "v" signals. The matrix to transfer between the RGB and the Y, u and v coordinate systems is as follows: EQU Y=0.299 R+0.587 G+0.114 B EQU u=0.493 (B-Y) and EQU v=0.877 (R-Y).
FIG. 2 shows a plan view of the chrominance in the Y, u, v coordinate system, with the luminance value shown in parenthesis. For example, the color blue would be located at +0.114 units of Y, +0.436 units of u and -0.1 units of V. In prior color correctors, a region 20 would be defined around a starting hue. In the example, the region surrounds the color red. In performing secondary color correction, the colors within the region 20 would be shifted to a region 22, the amount being defined by the desired amount of color change.
However, substantial problems have arisen in conventional color correction. Most notably, when the hue is changed, the luminance and saturation values for the first hue are not necessarily the same as for the second hue. The saturation is defined to be the length of the vector 24 from the Y, u and v coordinate system. By way of example, the luminance value of fully saturated blue is 0.114, whereas for cyan it is 0.701. The ratio of these two luminance values is 6.15. Thus, if a colorist changed the hue from blue to cyan, and the luminance gain was not increased by the ratio of 6.15, the cyan would appear very dim and unnatural looking. In operation, the colorist needs to correct the luminance gain for every hue change performed.
Additionally, to merely change the hue, without regard to the luminance, nonsensical results may be obtained. For example, the indication may be of "negative" color, or color percentages greater than 100%, neither of which can exist in nature.
No satisfactory solution to this long standing and vexing problem has been proposed heretofore.